3.5 - Composition Analysis - Determining Formulas

Question 1

How do chemists determine a formula of an unknown substance.

Answer 1

Determining the ratio between the mass of each element to the mass of the entire compound.

Question 2

what is the percentage by mass of each element in a compound?

Answer 2

Percentage composition?

Question 3

What does percentage composition compare?

Answer 3

Mass of part of a compound to mass of entire compound.

Question 4

What are the three steps to calculating a compound’s percentage composition?

Answer 4

• Determining the mass of the compound.
• Determine the total mass of each element.
• Determine the ratio those two as a percentage.

Question 5

Percentage composition gives you what information?

Answer 5

The simplest whole-number ratio by mass of the elements involved.

Question 6

Percentage composition gives you the simplest ratio of a chemical formula for which type of compounds?

Answer 6

Ionic compounds.

Question 7

T or F, ionic compounds can combine in multiple ratios.

Answer 7

False, they only combine in the simplest ratio.

Question 8

T or F, covalent compounds can combine in multiple ratios.

Answer 8

True, they can combine in different ratios.

Question 9

Which compound’s percentage composition doesn’t always match the actual ratio in the compound?

Answer 9

Covalent compounds.

Question 10

What are the two types of covalent formulas?

Answer 10

Empirical or Molecular.

Question 11

What do you call the smallest whole-number ratio between the elements in a molecular compound.

Answer 11

Empirical formula.

Question 12

What do you call the actual formula of a molecule, not always the simplest ratio.

Answer 12

Molecular formula.

Question 13

How do you find empirical formula from percentage composition? (no mass is given)

Answer 13

Assume you have 100 g of substance (easy # to work with), convert mass of each element to moles of each element, determine ratio between the moles of each element.

Question 14

When determining formula from percentage composition, do you count diatomic element’s molar mass twice?

Answer 14

No, you are calculating particles within the compound, not the element alone.

Question 15

How do you find the ratio between each atom when calculating empirical formula from percentage composition?

Answer 15

Divide each atom’s molar quantity by the smallest molar quantity.

Question 16

How do you find the molecular formula from empirical formula?

Answer 16

Emp. formula x compound’s molar mass / emp. molar mass